1. Process all ingredients, except sugar, together in a food processor until mixture resembles fine breadcrumbs.
2. Press mixture together until it forms a dough. Divide dough in two.
3. On a lightly flour-dusted board or bench roll out dough until 5mm thick.
4. Using a star-shaped cutter, cut shapes from dough. Transfer to prepared trays. Sprinkle with caster sugar if desired.
5. Bake at 160°C for 30 minutes or until dry and firm but still pale. Transfer to a wire rack to cool completely. Dust with icing sugar. Serve.